Conventional digital multicarrier systems transmit and receive digital signals by using a multiplicity of carriers or, respectively, subchannels having different frequencies. In this arrangement, a transmitter divides a transmit signal into a multiplicity of components, assigns the components to a particular carrier, codes each carrier in accordance with its component and transmits each carrier via one or more transmission channels.
The maximum amount of information which can be coded on a particular carrier is a function of the signal to noise ratio (SNR) with respect to the carrier. However, the signal to noise ratio of a transmission channel can depend on frequency so that a maximum amount of information which can be coded on a carrier differs from carrier to carrier.
In this context, the so-called bit loading method provides for selective assignment of respective bits to carriers or subchannels dependent upon a signal to noise ratio prevailing on the carrier. A bit loading algorithm supplies the values for a so-called bit allocation table (BAT) which specifies an amount of information to be coded for a respective carrier or, respectively, assigns this amount to it.
FIG. 1 shows a simplified block diagram of a line-connected multicarrier system according to the prior art. The multicarrier system essentially consists of a transmitter 1, a transmission medium or channel 2 and a receiver 3. If a band pass system is implemented, an RF modulation system 4 with an RF modulator 5 and an RF demodulator can also be optionally used.
According to FIG. 1, a series of input data to be transmitted are first converted into a parallel datastream, for example in a serial/parallel converter 10. The parallel datastream is then coded by a coding stage 11 dependent upon a bit allocation table at the transmitting end. More precisely, each carrier is assigned a signal space constellation which is dependent on a prevailing signal to noise ratio and is determined, or respectively, optimized as bit allocation table by a bit loading algorithm during an initialization or training phase in the conventional multicarrier system. The signal thus coded (in the frequency domain) is then shaped into suitable transmission pulses in a pulse shaper 12 and changed into a time domain by a time domain modulator 13 which generates a multicarrier signal. The multicarrier signal is then combined by a summer 14.
At the receiver 3 which is constructed symmetrically with respect to the transmitter 1 according to FIG. 1, the subchannels or carriers of the received signal or, respectively, of the input data values y′ are first separated, changed back into the frequency domain by a frequency domain modulator 16, supplied to a channel estimator 17 for estimating the characteristic properties of the transmission channel 2 and supplied to a decision stage 18 after a multiplicity of further processing stages, not shown. During the training phase, the decision stage input data values present in front of the decision stage 18 are derived and compared, for example by means of a noise variance determining device 7, with reference signals or reference data values RefX known at the receiving end, as a result of which a noise variance or noise power of the respective decision stage input data values Y is determined. In the same manner, these reference data values RefX are also supplied to the channel estimator 17 during an initialization phase as a result of which the characteristic properties such as, for example, a frequency response of the transmission channel 2 can be estimated. A bit allocation table 9 at the receiver end is written or adapted respectively, on the basis of this noise variance via a bit loading device 8 or, respectively, a bit loading algorithm executed therein. The bit allocation table 15 at the transmitting end is aligned with the bit allocation table 9 at the receiver end, for example via a return or control channel. The decision stage 18 is used for correlating the (inaccurate) decision stage input data value with an (exact) value of a predetermined set of values of a transmission format used (such as e.g. 4QAM). A subsequent decoding stage 19 then decodes the received data dependent upon the values of the bit allocation table 9 and a parallel/serial converter 20 converts the parallel datastream back into a serial output datastream.
In this manner, a bit allocation table optimized for a signal to noise ratio during the initialization phase or training phase is obtained which allows minimum bit error rates to be achieved for the output data (values). The disadvantageous factor in such a conventional multicarrier system is, however, the fact that this results in an unnecessary reduction in bandwidth. Conventional devices and methods for increasing a bandwidth in a line-connected multicarrier system have the disadvantage, however, that they are technically elaborate and thus expensive and require high computing power.